U.S. Pat. No. 4,569,590 to Karney describes a system for determining unknown index of refraction of a sample fluid with reference to the refraction index of a known reference fluid. The system comprises a collimated light source, a light pervious cell formed as a biconvex lens fillable with a fluid, a pair of gratings and a screen. Collimated light is directed through the cell filled first with the reference fluid and then with the sample fluid, onto the gratings, and reference and sample patterns are obtained thereby on the screen. The sample pattern will be different from the reference one if the optical power of the biconvex lens changes due to the change of the liquid in the cell. The difference between the reference pattern and the sample pattern is then measured and used for calculating the refraction index of the sample fluid.
EP 1324015 describes similar techniques for measuring optical parameters of a phase object based on recording a moiré pattern viewed through the phase object. The moiré pattern is formed by illuminating two gratings by diffuse light, and projecting their images on a screen through the phase object. The optical parameters of the phase object are calculated from the moiré pattern.
Such methods require relatively complex optical systems, recording and measurement of images and sophisticated calculations in order to determine accurately a change in the refraction index of the liquid.
Co-pending International Patent Application PCT/IL2003/000987 discloses, inter alia, methods systems and devices for identification of changes in fluids.